Composite articles separating mercury from fluids

ABSTRACT

Composite articles are useful for separating mercury from fluids. The composite articles can be porous supports comprising an inert substrate having immobilized thereon finely divided gold optionally in combination with a tin salt coating. The porous support can be a particulate or porous fibrous webs. Alternatively, the composite articles can comprise a porous fibrous membrane having enmeshed therein the aforementioned porous supports which can be in particulate or fibrous forms. The method for separating elemental, ionic, or organic mercury in fluids comprises the step of contacting and passing a fluid containing mercury through a support comprising a porous, high surface area, inert substrate on which is immobilized finely divided elemental gold at a controlled rate for a time sufficient for the mercury to sorb to the elemental gold and to provide an immobilized gold-mercury amalgam on the support. If a tin salt also is immobilized on the inert substrate, mercury-tin salt can also be formed. In a further, and optional, step, elemental mercury can be removed from the support and optionally can be quantified.

This is a division of application Ser. No. 08/268,286 filed Jun. 29,1994, now U.S. Pat. No. 5,492,627.

TECHNICAL FIELD

This invention relates to composite articles for use in concentrating orseparating mercury in fluids.

BACKGROUND OF THE INVENTION

Currently a good deal of interest has been generated in the analyticalcommunity in particle loaded membrane technology and its applicationsfor solid phase extractions as discussed by Hagen et al., "MembraneApproach to Solid Phase Extractions", Analytica Chimica Acta, 236,157-164, 1990, and by Markell et al., "New Technologies in Solid PhaseExtraction", LC/GC, Volume 9, Number 5, 1991. This technology has beenshown to be useful for isolation of hydrophobic organic pollutants byadsorptive interactions and has demonstrated the advantages of fastdiffusion kinetics when small, high surface area particles are packedclosely together in uniform membranes with little or no channeling andwith controlled porosity. Van Osch et al. have described a membrane(i.e., a "pellet impervious to a solution of ions) for ion electrodes,Z. Anal. Chem. 271-4, 1975. Solid "membranes" disclosed in U.S. Pat. No.3,824,169 are imporous (non-porous), chemically inert, composites ofgold and salts pressed into pellets which are used in potentiometricelectrode technology. Solid "membranes" are distinct from porousparticle loaded articles disclosed in U.S. Pat. Nos. 4,153,661,4,460,642, 4,810,381, 4,906,378, 4,971,736, 5,019,232, 5,071,610, and5,147,539 for applications in separation science utilizing solid phaseextractions.

Particle-loaded, non-woven, fibrous articles wherein the non-wovenfibrous web can be compressed, fused, melt-extruded, air-laid,spunbonded, mechanically pressed, or derived from phase :separationprocesses have been disclosed as useful in separation science. Sheetproducts of non-woven webs having dispersed therein sorbent particulatehave been disclosed to be useful, for example, in respirators,protective garments, fluid-retaining articles, and as wipes for oiland/or water, and as chromatographic and separation articles. Coatedinorganic oxide particles have also been enmeshed in such webs.

Contamination of fluids by mercury has been a long-standingenvironmental concern. Preconcentration or separation of mercury byamalgamation with gold, as a precursor to quantitative analysis of themercury, has been described. U.S. Pat. No. 3,849,533 discloses powderedcarrier material impregnated with a noble metal salt, such as, e.g.,silver nitrate, as an absorption medium for mercury. Gold-coated seasand has been described as a mercury trap in Analytica Chimica Acta 107pp. 159-167 (1977), as has gold-coated powdered pumice (Japan Kokai 55084.536, Derwent Abstract). A gold-coated fritted glass disk has beendescribed for mercury collection in Analytical Chemistry 43, pp. 1511-2(1971). Particulate material is typically described in connection withan extraction column apparatus in which mercury vapor ormercury-containing liquid are passed over the absorbing particulate in acolumn, after which the absorbing particulate is heated to drive offmercury for direct analysis by, e.g., atomic absorption spectrometry(AAS). In the case of the glass disk, the disk itself was heated todrive off mercury for analysis.

Other references in which gold-coated supports have been used todetermine mercury include Analytica Chimica Acta 106, pp 405-410 (1979),Analytica Chimica Acta 220, pp. 257-261 (1989), Chemical Abstracts 111:83762j (1989), U.S. Pat. Nos. 5,322,628, 5,271,760 and 4,892,567.

SUMMARY OF THE INVENTION

Briefly, the present invention provides a novel method for separatingmercury from fluids comprising contacting and passing a fluid comprisingmercury (elemental, ionic, or organic mercury) through a porous supportwhich can be particulate, coated particulate, coated fiber, or any otherporous, high surface area support, on the surface of which isimmobilized finely divided elemental gold optionally in combination witha tin salt coating.

The supports of the invention can be particulate or fibers that can bepacked in a column and challenged with a fluid comprising mercury as ananalyte in order to separate or concentrate the mercury.

In another aspect, the present invention provides the supports disclosedabove (in particulate or fiber form) entrapped in a porous, nonwovenfibrous web. These webs can be at least one of polyamide, PTFE,polyolefin, glass, ceramic, or quartz. Preferably, the supports(particulate or fibers) are enmeshed in a porous polytetrafluoroethylene(PTFE) fibrillated matrix. The PTFE composite matrix provides an easilyhandleable article for use in the method of the present invention.

Preparation of porous supports on which finely divided gold isimmobilized may be provided by directly coating the support withelemental gold (e.g., by sputtering) or by coating a porous inertsupport with a gold salt (preferably gold chloride) solution. The coatedgold salt is then reduced to elemental gold by methods well known in theart. See, for example, W. Romanowski, "Highly Dispersed Metals", JohnWiley & Sons, New York (1987) 52-57. Preferred methods for reducingionic gold to elemental gold include treatment with formaldehydesolution or stannous salt (preferably stannous chloride) solution.

In a preferred embodiment, a PTFE fibrillated matrix having reactive orsorptive supports enmeshed therein is made by well-known methods.Preferably, it is made by the method disclosed in U.S. Pat. No.5,071,610, Example 1, which is incorporated herein by reference forpreparation of PTFE fibrillated matrices.

In a further aspect, the particulate comprising immobilized gold andoptionally a tin salt can be used in a method of concentrating orseparating elemental mercury from fluids, the method comprising the stepof a) contacting and passing a fluid comprising at least one ofelemental and ionic mercury as an analyte through a porous supportcomprising a porous, high surface area substrate on which is immobilizedfinely divided elemental gold and optionally a tin salt to provide agold-mercury amalgam and optionally a mercury-sorbed tin salt.

Where the mercury is in the form of ionic mercury (i.e., Hg⁺ or Hg⁺⁺),the method can further comprise the step of b) converting ionic mercuryto elemental mercury prior to or simultaneous with step a) above.

Where the mercury is in the form of organic mercury (i.e., R-Hg, whereinR is an organic group that covalently bonds to Hg), the method canfurther comprise the step of converting organic mercury to ionic mercuryprior to step b) above. One method includes conversion of organicmercury to ionic mercury using, for example, permanganate oxidation.See, for example, F. A. J. Armstrong et al., ATOMIC ABSORPTIONNEWSLETTER 10(5), September-October 1971.

In a still further aspect, the elemental mercury can be recovered fromthe resulting gold-mercury amalgam for further characterization. Thiscan be accomplished by dissolving the gold-mercury amalgam and theoptionally present mercury-sorbed tin salt with aqua regia or byheating.

In a yet further aspect, the present invention provides a supportcomprising an inert, porous, high surface area substrate, as previouslydescribed, on which is immobilized finely divided elemental gold-mercuryamalgam. The supports can be packed in a column or enmeshed in a porousfibrous membrane, as described herein. In this aspect, there can beprovided analytical standards wherein known levels of mercury arepresent for calibration purposes using novel supports comprising aninert, porous, high surface area substrate, as previously described, onwhich is immobilized finely divided elemental gold-mercury amalgam.

In this application:

"immobilized gold" means gold that is tightly bound to a poroussubstrate so that the gold cannot be mechanically removed, e.g., bywashing; the gold is finely divided and under an electron microscopeappears as "islands" or "domains" having an approximate size (largestdimension or largest diameter) generally in the range of 1 to 100 nm,preferably 5 to 50 nm, more preferably 10 to 20 nm;

"immobilized gold-mercury amalgam" means a solid solution of mercury andgold which is tightly bound to a porous substance; the gold-mercuryamalgam is finely divided and under an electron microscope appears as"islands" or "domains" having an approximate size (largest dimension)generally in the range of 1 to 100 nm, preferably 5 to 50 nm, morepreferably 10 to 20 nm.

"void volume" means the volume of the vacancies in the structure of acomposite;

"web" or "membrane" means a porous sheet material that can be fibrous ornonfibrous; preferably it has a void volume in the range 30 to 80percent, preferably 55 to 65 percent, with a pore size of 0.4 to 2.5micrometers (4 to 25×10² nm), preferably 0.6 to 0.8 micrometers (6 to8×10² nm);

"matrix" means an open-structure entangled mass of microfibers;

"particles" or "particulate" means porous inert substrates with solidshapes (not including PTFE) preferably having an average diameter 0.1 to200 micrometers, more preferably an average diameter of 1 to 40micrometers, even more preferably an average diameter of 5 to 25micrometers, and most preferably an average diameter of 10-15micrometers, with an aspect ratio of 1 to 1,000,000; and

"porous support" means a porous particle, a collection of porousparticles, or a porous web.

As noted above, the articles can comprise porous fibrous membranes whichpreferably comprise a polytetrafluoroethylene (PTFE) fibril matrixhaving enmeshed therein porous supports (particles), or they cancomprise packed beds (columns) of the porous supports. The articles canbe used in a method of concentrating or separating mercury (elemental,ionic, or organic) from fluids, as described above.

The present invention method for separating mercury from a fluid hasbeen found to be advantageous over conventional methods which useparticles in a bed or cartridge in that there is achieved efficientadsorption of mercury at high flow rates, with less plugging (because oflarger surface area gold-coated particles), and faster mass transfer.The high surface area porous particulate of very small average size,preferably enmeshed in a porous web, makes these desirable achievementspossible. Additionally, efficiency is enhanced because of low pressuredrop in particle-loaded webs. When inert porous matrices, such aspolytetrafluoroethylene are used, dissolution of gold and mercury can beaccomplished by aqua regia, or preferably, modified aqua regia in whichhydrochloric acid is replaced by hydrobromic acid. Use of modified aquaregia overcomes problems of mercury chloride adhering to solid surfacesin the analytical apparatus, when dissolution takes place. The mercurybromide solution overcomes the well-known problems associated withmercury salt deposition in the analytical apparatus when usingInductively Coupled Argon Plasma Emission Spectroscopy (ICP). In manycases, fibrous webs or membranes cannot withstand treatment with aquaregia or modified aqua regia. The gold and mercury can also be desorbedby high pH cyanide solution. In such a case a minimal amount of highsurface area gold on high surface area particulate may be desirable sothat low concentrations of cyanide can be used. When the novelparticulate is in a bed or incorporated in a porous, fibrous web,mercury alone can be desorbed by heating, provided that when the porous,fibrous web is present, it is stable at the elevated temperaturesrequired for the mercury desorption. In such cases the gold coatedparticles in a bed or particle-loaded web can be reused.

The gold-coated particulate (in or out of a web) is advantageous oversimple gold coated sand or gold films because of the high surface areawhich is provided. This allows faster and more efficient adsorption ofelemental mercury. Alumina or zirconia supports are preferred whendesorption is desired because they are essentially inert to aqua regiaor cyanide at high pH and because they can be prepared with high surfacearea. Use of low or ambient temperatures for the desorption processallows use of certain temperature-sensitive webs for enmeshinggold-coated particles.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In this invention, interactions on a solid phase are utilized to convertinsoluble elemental gold in a bed of supports or in a kineticallyoptimized composite porous web to a gold-mercury amalgam. Advantage istaken of rapid diffusion kinetics to perform fast sorptive processes,namely the amalgamation of mercury with gold. Amalgamation, as is knownin the art, is the forming of a solid solution of mercury with anothermetal, in this case gold. In the method of the invention, a sample fluidcomprising mercury as an analyte is passed through a web or columncomprising the gold coated supports of the invention.

Excessive amounts of gold should be avoided so that the method can beeconomically feasible. Preferably, porous supports can contain in therange of about 0.05 to 10% by weight of gold compared to weight ofsupport, more preferably 0.5 to 6 weight percent, and most preferably 1to 3 weight percent. This range can be extended for certainapplications. Preferably, the immobilized high surface area gold has asurface area at least in the range of 5 to 100 m² /g.

In one embodiment, the gold containing particulate is packed into bedsor columns through which fluids containing mercury as an analyte arepassed to convert the immobilized elemental gold into immobilizedgold-mercury amalgam. More particularly, a high surface area particulatesubstrate such as silica, alumina, or zirconia, is coated preferablywith an alcohol solution (e.g., using a combination HCl/methanol/hexanesolvent) of gold chloride (preferably 1 to 10 weight percent or up tosaturation of the solution), solvent is removed and gold chloride issubsequently reduced to elemental gold. Preferably, the particulatesubstrate has a surface area in the range of 20 to 800 m² /g, morepreferably, 100 to 350 m² /g. The resulting supports can be used in themethod of the invention in beds or cartridges or the supports can thenbe embedded in a fibrous web which preferably is a fibrillated PTFEmembrane composite using the method disclosed for example, in U.S. Pat.No. 5,071,610, Example 1. In a second embodiment, a high surface area,uncoated, particulate (such as silica) loaded web is prepared accordingto methods disclosed in, for example, U.S. Pat. No. 5,071,610,Example 1. The enmeshed particulate in the composite web cansubsequently be treated with a solution of a gold salt, such as goldchloride, and the gold salt which coats the surface and internal poresof the particulate is then reduced to metallic gold in-situ. In eithercase, it is desirable to use small high surface area particles which canbe uniformly enmeshed in the matrix with controlled interstitialporosity.

In a second embodiment of the invention wherein the challenging fluidcontains ionic mercury (Hg⁺¹ or Hg⁺²), reduction of mercury to elementalmercury and amalgamation can take place in a single step. This is madepossible by sorbing a stannous salt, such as stannous chloride, onto atleast a portion of the gold containing particulate. Ionic mercury in thechallenging fluid is sorbed directly by the tin salt or reduced toelemental mercury which is then immobilized by formation of gold-mercuryamalgam on the particulate. This one-step method eliminates thepossibility of mercury vapor forming and losses due to its volatility.Useful amounts of stannous salt can be obtained, for example, bytreatment with aqueous 5 percent by weight stannous chloride solution.In some cases, it may be desirable to treat the base particulate, priorto coating with gold, with a tin (preferably stannous) salt solution toreduce any ionic gold present to elemental gold.

A preferred method for preparing the preferred PTFE composite article ofthe invention comprises the steps of:

a) admixing lubricant (preferably water) with a blend comprising porousparticulate and polytetrafluoroethylene (PTFE) particles to form a softdough-like mass, the lubricant being present in an amount to exceed thesorptive capacity of the particulate by at least three weight percent,said mass having a cohesive consistency, and the ratio of particulate toPTFE preferably being in the range of 40:1 to 1:4;

b) intensively mixing said mass at a temperature and for a timesufficient to cause initial fibrillation of said PTFE particles;

c) biaxially calendering said mass between gaps in calendering rollsmaintained at a temperature and for a time, while closing the gapbetween the calendering rolls with each successive calenderingoperation, to cause additional fibrillation of said PTFE particles toform a self-supporting sheet having a void volume in the range of 30 to80 percent and a mean pore size in the range of 0.3 to 5.0 micrometers,wherein said void volume and mean pore size vary directly with and arecontrolled by the amount of lubricant present during processing.

More particularly, preparation of porous fibrous webs for entrapment ofsupports of the invention therein, can be as follows:

A. PTFE Membranes (Webs)

In one embodiment of the article of the present invention, an aqueousPTFE dispersion is used to produce a fibrillated web. This milky-whitedispersion contains about 30% to 70% (by weight) of minute PTFEparticles suspended in water. A major portion of these PTFE particlesrange in size from 0.05 μm to about 0.5 μm. Commercially availableaqueous PTFE dispersions may contain other ingredients such assurfactants and stabilizers which promote continued suspension. Examplesof such commercially available dispersions include Teflon™ 30, Teflon™30B, and Teflon™ 42 (DuPont de Nemours Chemical Corp.; Wilmington,Del.). Teflon™ 30 and Teflon™ 30B contain about 59% to 61% (by weight)PTFE solids and about 5.5% to 6.5% (by weight, based on the weight ofPTFE resin) of a non-ionic wetting agent, typically octylphenylpolyoxyethylene or nonylphenyl polyoxyethylene. Teflon™ 42 containsabout 32% to 35% (by weight) PTFE solids and no wetting agent (but doescontain a surface layer of organic solvent to prevent evaporation).

The composite sheet article comprising fibrillated PTFE preferably isprepared as described in any of U.S. Pat. Nos. 4,153,661, 4,460,642, and5,071,610, the processes of which are incorporated herein by reference,by blending the desired reactive supports into the aqueous PTFE emulsionin the presence of sufficient lubricant to exceed the absorptivecapacity of the solids yet maintain a putty-like consistency. Thisputty-like mass is then subjected to intensive mixing at a temperaturepreferably between 40° and 100° C. to cause initial fibrillation of thePTFE particles. The resulting putty-like mass is then repeatedly andbiaxially calendered, with a progressive narrowing of the gap betweenthe rollers (while at least maintaining the water content), until shearcauses the PTFE to fibrillate and enmesh the particulate and a layer ofdesired thickness is obtained. Removal of any residual surfactant orwetting agent by organic solvent extraction or by washing with waterafter formation of the sheet article is generally desirable. Theresultant sheet is then dried. Such sheets preferably have a thicknessin the range of 0.1 mm to 0.5 mm. Sheet articles with a thickness in thegeneral range of 0.05 mm to 10 mm can be useful.

The void size and volume within such a membrane can be controlled byregulating the lubricant level during fabrication as described in U.S.Pat. No. 5,071,610. Because both the size and the volume of the voidscan vary directly with the amount of lubricant present during thefibrillation process, webs capable of entrapping particles of varioussizes are possible. For instance, increasing the amount of lubricant tothe point where it exceeds the lubricant sorptive capacity of theparticulate by at least 3% (by weight) and up to 200% (by weight) canprovide mean pore sizes in the range of 0.3 μm to 5.0 μm with at least90% of the pores having a size of less than 3.6 μm. This process can beused to create a web with supports enmeshed therein. The PTFE whichforms the web within which particulate is to be trapped can be obtainedin resin emulsion form wherein the PTFE and lubricant are alreadypre-mixed (e.g., Teflon™ 30 or 30B, DuPont de Nemours; Wilmington,Del.). To this emulsion can be added additional lubricant in the form ofwater, water-based solvents such as a water-alcohol solution, oreasily-removable organic solvents such as ketones, esters, and ethers,to obtain the aforementioned desired proportion of lubricant andparticulate.

B. Non-PTFE Membranes (Webs)

In other embodiments of the present invention, the fibrous web cancomprise non-woven, polymeric macro- or microfibers preferably selectedfrom the group of polymers consisting of polyamide, polyolefin,polyester, polyurethane, polyvinylhalide, or glass, ceramic, or quartzfibers, or a combination thereof. If polyvinylhalide is used, itpreferably comprises fluorine of at most 75% (by weight) and morepreferably of at most 65% (by weight). Addition of a surfactant to suchwebs may be desirable to increase the wettability of the componentfibers.

1. Macrofibers

The web can comprise thermoplastic, melt-extruded, large-diameter fiberswhich have been mechanically-calendered, air-laid, or spunbonded. Thesefibers have average diameters in the general range of 50 μm to 1000 μm.

Such non-woven webs with large-diameter fibers can be prepared by aspunbond process which is well known in the art. (See, e.g., U.S. Pat.Nos. 3,338,992, 3,509,009, and 3,528,129, the fiber preparationprocesses of which are incorporated herein by reference.) As describedin these references, a post-fiber spinning web-consolidation step (i.e.,calendering) can be required to produce a self-supporting web.Spunbonded webs are commercially available from, for example, AMOCO,Inc. (Napierville, Ill.).

Non-woven webs, made from large-diameter staple fibers can also beformed on carding or air-laid machines (such as a Rando-Webber™, Model12BS made by Curlator Corp., East Rochester, N.Y.), as is well known inthe art. See, e.g., U.S. Pat. Nos. 4,437,271, 4,893,439, 5,030,496, and5,082,720, the processes of which are incorporated herein by reference.

A binder is normally used to produce self-supporting webs prepared bythe air-laying and carding processes and is optional where the spunbondprocess is used. Such binders can take the form of resin systems whichare applied after web formation or of binder fibers which areincorporated into the web during the air laying process. Examples ofsuch resin systems include phenolic resins and polyurethanes. Examplesof common binder fibers include adhesive-only type fibers such as Kodel™43UD (Eastman Chemical Products; Kingsport, Tenn.) and bicomponentfibers, which are available in either side-by-side form (e.g., Chisso ESFibers, Chisso Corp., Osaka, Japan) or sheath-core form (e.g., Melty™Fiber Type 4080, Unitika Ltd., Osaka, Japan). Application of heat and/orradiation to the web "cures" either type of binder system andconsolidates the web.

Generally speaking, non-woven webs comprising macrofibers haverelatively large voids. Therefore, such webs have low capture efficiencyof small-diameter particulate which is introduced into the web.Nevertheless, particulate can be incorporated into the non-woven webs byat least four means. First, where relatively large particulate is to beused, it can be added directly to the web, which is then calendered toactually enmesh the particulate in the web (much like the PTFE websdescribed previously). Second, particulate can be incorporated into theprimary binder system (discussed above) which is applied to thenon-woven web. Curing of this binder adhesively attaches the particulateto the web. Third, a secondary binder system can be introduced into theweb. Once the particulate is added to the web, the secondary binder iscured (independent of the primary system) to adhesively incorporate theparticulate into the web. Fourth, where a binder fiber has beenintroduced into the web during the air laying or carding process, such afiber can be heated above its softening temperature. This adhesivelycaptures particulate which is introduced into the web. Of these methodsinvolving non-PTFE macrofibers, those using a binder system aregenerally the most effective in capturing particulate. Adhesive levelswhich will promote point contact adhesion are preferred.

Once the particulate has been added, the loaded webs are typicallyfurther consolidated by, for example, a calendering process. Thisfurther enmeshes the particulate within the web structure.

Webs comprising larger diameter fibers (i.e., fibers which averagediameters between 50 μm and 1000 μm) have relatively high flow ratesbecause they have a relatively large mean void size.

2. Microfibers

When the fibrous web comprises non-woven microfibers, those microfibersprovide thermoplastic, melt-blown polymeric materials having sorptive oractive particulate dispersed therein. Preferred polymeric materialsinclude polyolefins such as polypropylene and polyethylene, preferablyfurther comprising a surfactant, as described in, for example, U.S. Pat.No. 4,933,229, the process of which is incorporated herein by reference.Alternatively, surfactant can be applied to a blown microfibrous (BMF)web subsequent to web formation. Particulate can be incorporated intoBMF webs as described in U.S. Pat. No. 3,971,373, the process of whichis incorporated herein by reference.

Microfibrous webs of the present invention have average fiber diametersup to 50 μm, preferably from 2 μm to 25 μm, and most preferably from 3μm to 10 μm. Because the void sizes in such webs range from 0.1 μm to 10μm, preferably from 0.5 μm to 5 μm, flow through these webs is not asgreat as is flow through the macrofibrous webs described above.

3. Porous membranes can be provided by methods known in the art. Suchporous membranes can be, for example, polyolefin,, including PTFE andpolypropylene, and polyamide, polyester, and glass, quartz, or ceramicfibers, or any combination of the foregoing, which porous membranes canbe coated with finely divided gold by any of the methods, includingsputtering and gold salt reduction, known in the art to provide porousreactive supports of the invention. Porosity of membranes before andafter immobilization of gold is sufficient to allow passage of fluidscontaining mercury.

In each of these methods, diffusion and reaction kinetics determine therates at which insoluble, elemental gold is transformed into theinsoluble gold-mercury amalgam. The time for diffusion is a function ofthe distance an analyte must migrate before contacting a particle and isestimated by Equation 1 where t_(d) is the diffusion time, d is thedistance between particles, and D is the diffusion coefficient for thefluid involved.

    t.sub.d =d.sup.2 /2D                                       Equation 1

To optimize these criteria we investigated: (1) incorporation ofmetallic gold using 99.95% purity, 2-5 micrometer spherical goldparticulate obtained from Johnson Matthey Co., Ward Hill, Mass., (2)sputtering gold onto high surface area metal oxide substrates such assilica, zirconia, or the gold can be sputtered onto any porous substratesuch as polyamide, PTFE, polyolefin (preferably polypropylene). A sampleof gold sputtered coated particulate was prepared using 8-10 micrometersilica (Varian Associates, Harbor City, Calif.), and (3) coatingmethanolic solutions of gold chloride (Aldrich Chemical Co., MilwaukeeWis.) onto these substrates with subsequent reduction to metallic gold.This latter approach using silica or other sorptive particulate coatedwith gold chloride and subsequently, employing a reduction step(preferably using formaldehyde as reducing agent) to obtain elementalgold is the preferred method for this application. The method of theinvention utilizes immobilized gold containing particles within acomposite membrane or particle packed columns.

The method of the invention relating to separating mercury from fluidscan be practiced using devices such as passive or dynamic air monitoringbadges or cassettes.

Objects and advantages of this invention are further illustrated by thefollowing examples, but the particular materials and amounts thereofrecited in these examples, as well as other conditions, and details,should not be construed to unduly limit this invention.

EXAMPLES Example 1

In this example, Sample 1 was prepared by dissolving 300 mg of AuCl₃(Auric Chloride, Aldrich Chemical Co., Milwaukee, Wis.) in 20 mL ofmethanol and this solution was mixed with 10 grams of chromatographicgrade 8-10 micrometer diameter silica particulate, 60 to 80 Å internalpore (Varian Associates, Harbor City, Calif.). The resultant slurry wasstirred for five minutes to insure adequate coating of the silica withthe AuCl₃ solution. The slurry was then transferred to a porcelaincrucible and heated at 40°-50° C. until most of the methanol was removedby evaporation. The crucible was then heated to redness (approximately600° C.) and allowed to cool to room temperature. The coated particulateobtained was a relatively uniformly coated, purple colored particulatecontaining 2% by weight gold. The purple coloration was indicative offinely divided colloidal gold. A composite membrane comprising 10% PTFEand 90% by weight of the gold coated silica was then prepared asdescribed for sorptive particulate in U.S. Pat. No. 5,071,610.

Example 2

In this example, Sample 2 was prepared using zirconia as the substratefor the gold coated particulate because of its greater stability inbasic solutions. Forty grams of zirconia particulate, about 8 micrometersize (prepared as disclosed in U.S. Pat. No. 5,015,373, Example 5) wereplaced in a 500 mL round bottom flask to which was added a solution of1.8 grams of AuCl₃ in 200 mL of 0.15 M HCl in anhydrous methanol. (Thiscorresponds to a 3% by weight loading of gold on the zirconiaparticulate). The flask was shaken to ensure good mixing and themethanol was then removed in a flash vacuum evaporator with constanttumbling. The resulting gold chloride coated zirconia powder was a paleorange color. The gold chloride coated zirconia powder was then treatedwith 70 mL of 6% sodium borohydride in water to reduce the ionic gold toelemental gold. A vigorous reaction resulted and the powder turnedalmost black indicating an efficient conversion of ionic gold tometallic colloidal gold. The coated particulate was allowed to settleand was washed once with. 100 mL of distilled water followed by two 300mL methanol washes and one 300 mL wash with methyl t-butyl ether. Theslurry was transferred to an evaporating dish and brought to completedryness at 40°-50° C. The resulting gold coated particulate had a lightpurple color. Membranes were prepared with this particulate as describedin U.S. Pat. No. 5,071,610, see Example 1.

Example 3

In this example, Sample 3 was prepared by pretreating 30 grams ofzirconia (same as used in Example 2) with phosphate. The zirconia wasplaced in a centrifuge bottle and mixed with 100 mL of 10% (w/w)phosphoric acid in water. This was centrifuged to isolate the solidparticulate which was then washed with 500 mL of water followed by 200mL of 100 grams/liter trisodium phosphate. The resulting powder was thenwashed twice with 500 mL of water and once with 500 mL of methanol. Theexcess methanol was removed and the zirconia slurry transferred to a 500mL round bottom flask. A solution of 2.3 grams of auric chloride in 400mL of 90% methanol in water was added to the zirconia slurry in theflask. This was thoroughly mixed and the solvent was removed by flashevaporation. Two grams of sodium borohydride was then dissolved in 100mL of 70% methanol and immediately mixed with the dry gold coatedzirconia powder. A vigorous reaction took place and the zirconia powderturned very dark. The excess liquid was removed by decanting and theproduct washed once with water, once with methanol and once with acetone(500 mLs each). Excess solvent was removed by evaporation at low heatand the resulting dark purple powder contained 5% by weight elementalgo].d. Membranes were prepared with this particulate as described inU.S. Pat. No. 5,071,610, Example 1. This trial showed that the phosphatetreated zirconia was able to adsorb higher levels of gold chloride thanthe bare zirconia.

Example 4

In this example, Sample 4 was prepared by placing 20 grams of silica(same as in Example 1) in a round-bottom flask and mixing with asolution of 1 gram of auric chloride in 200 mL of 0.02 M HCl inmethanol. The mixture was evaporated to dryness and the resulting lightorange powder was mixed with 1 gram of calcium hydroxide powder (thiscaused the resulting mixture to turn grayish). A portion of this powdermixture was heated in a 5% hydrogen, 95% argon atmosphere to 250° C. fortwenty minutes. The resulting powder was dark purple to black in colorindicating a high level of colloidal gold was deposited on the silicaparticulate. The second portion of Sample 4 was heated to 450° C. for 20minutes in this hydrogen atmosphere. This resulted a pinkish coloredpowder. Membranes were prepared with this particulate as described inU.S. Pat. No. 5,071,610.

Example 5

Approximately 10 grams of silica (8-10 micrometer diameter particulate,Varian Associates, Harbor City, Calif.) were placed in the lid of aplastic petri dish which was inserted into a Hummer VII Sputter Coater(Anatech Ltd., Alexandria, Va.). The coater was set up to coat 30 nmthickness at a rate of 4 nm/min. The coater did not have a way tomeasure thickness; rather, it used the current in the plasma taken by atime factor to determine thickness. After the 30 nm coating, theparticulate was agitated to expose new surfaces. This process wasrepeated until 90 nm was added to the silica in the petri dish lid. Theresulting particulate had a purple coloration typical of finely dividedelemental gold.

Example 6

Commercially available porous filtration webs (Cole-Parmer, Chicago,Ill.) were sputter-coated on the same apparatus described in Example 5.In this case, only one side of the web was coated. These webs were inthe form of disks, 47 millimeter in diameter and had pore sizes of 0.2micrometer. The webs coated were nylon, polytetrafluoroethylene (PTFE),and polypropylene. The coating thickness on all the webs was 90nanometers. An additional PTFE web was coated to a gold thickness of 30nanometers. The coated webs had the appearance of metallic gold in coloron the coated side only. This is in contrast to the purple coloration ofthe material prepared for Example 5 because of difference in goldparticle size. Similar webs can be prepared using any of a variety ofcommercially available filtration webs.

To evaluate the porosity, the webs were mounted in a 25 mm Milliporefiltration (Millipore Corp., Marlborough, Mass.) apparatus. Usingvacuum, methanol and water were pulled through the webs, demonstratingporosity.

Example 7

In this example, 50 milligrams of gold chloride were dissolved in 2 mLs1.5 molar hydrochloric acid in methanol. Six mLs of isopropanol wereadded to this solution followed by 800 milligrams of acidic alumina (75to 150 micrometer diameter, Bio Rad Inc., Hercules, Calif.). Sixty mLsof hexane were then added and the slurry was mixed thoroughly. Theprecipitate was then separated and air dried. The dry particulate wasthen placed in a flask and heated to 170° C. under a stream of hydrogengas. The purple colored particulate was then washed several timessequentially with water, methanol, and acetone. This particulate wasthen air dried.

Example 8

In this example 80 milligrams of gold chloride were dissolved in 2 mLsof 1.5 molar hydrochloric acid in methanol. Six mLs of isopropanol wereadded to this solution followed by 800 milligrams of 100 micrometerdiameter silica particulate with 300 Angstrom internal pores (DAVISIL™,W. A. Grace Inc., Baltimore, Md.) Sixty mLs of hexane were then addedand the mixture thoroughly mixed. The gold chloride coated particulatewas then separated from the liquid phase and air dried. It was thenheated to 170° C. under a stream of hydrogen which reduced the ionicgold species to elemental gold resulting in a deep purple coloredparticulate. This particulate was then washed several times sequentiallywith water, methanol, and acetone followed by air drying.

Example 9

Mercury in Air Samples: Passive Sampling; gold-coated silica particles

Gold-coated silica particulate, prepared according to Example 8, wasincorporated into a composite web comprising 10% PTFE and 90%particulate, as described for sorptive particulate in U.S. Pat. No.5,071,610. Four 33-mm diameter disks of the web were placed in a passivesampling diffusion cassette (Model 3500 Organic Vapor Monitor, 3M, St.Paul, Minn.) and the cassettes were placed in a standard mercury vaporgenerating apparatus (Am. Ind. Hyg. Assoc. J. 38 378 (1977)) for 290min.

Each of the four webs, along with a non-exposed blank web, was treatedwith a 3:1 mixture of hydrobromic acid (48% aq. solution): concentratednitric acid as follows: Each disk was placed in a 47 mm diameterMillipore extraction apparatus that had been adapted to hold the 33 mmdiameter sample disks without seepage around the edge of the disks. Asmall amount of the HBr:HNO₃ was placed on the sample for 3 minutes,then pulled through the disk under aspirator vacuum. The disk was washedwith a small amount of water, and the combined washings were diluted fordirect analysis of mercury using a model ARL-3410 Inductively CoupledPlasma (ICP) spectrometer (Applied Research Laboratories, Valencia,Calif.) equipped with a Cetac ultrasonic nebulizer, measuring at 194.227nm. Results of the analyses are presented in Table 1, below.

                  TABLE 1                                                         ______________________________________                                        Sample      μg Hg Recovered*                                               ______________________________________                                        1           1.298                                                             2           1.325                                                             3           1.098                                                             4           1.300                                                             Blank 1     0.423                                                             ______________________________________                                         *Theoretical Hg available = 1.466 μg                                  

Example 10

Mercury in Air Samples: Dynamic Sampling; gold coated silica particles

A microporous PTFE web, as described in Example 9, was cut into 25 mmdiameter disks and disks were inserted into 25 mm diameter MilliporeSwinnex™ cassettes adapted for active air sampling. Two of the cassetteswere connected in parallel so that air being sampled passed through twoparticle-loaded webs before exiting the second cassette. Three sets oftwo cassettes were assembled and subjected to air containing mercuryvapor generated by the apparatus described in Example 9. Air from themercury vapor generator was pulled through the two cassettes withaspirator vacuum at the rate shown in Table 2. A pair of webs wasassembled in a fourth set of cassettes as blanks. Each of the webs waseluted with HBr:HNO₃, as described above, and the amount of mercury ineach web was determined by ICP spectrometry. Results of the test areshown in Table 2. In each case, mercury-loaded air impinged on Sample Abefore Sample B.

                  TABLE 2                                                         ______________________________________                                                                    Theoretical                                                                           Measured                                          Time,   Flow rate,  Hg/sample,                                                                            Hg/sample,                                Sample  min.    L/min.      μg   μg                                     ______________________________________                                        5A      10      1.08        1.05    1.16                                      5B      10      1.08        --      0.36                                       5B*    10      1.08        --      0.09                                      6A      20      0.98        1.90    1.69                                      6B      20      0.98        --      0.30                                      7A      30      1.02        2.97    2.42                                      7B      30      1.02        --      0.27                                      Blank A 0       0           --      0.43                                      Blank B 0       0           --      0.27                                      ______________________________________                                         *second eluate of 5B.                                                    

Data in this table shows that essentially all of the mercury in the airsamples was retained on the first membrane, and that from 80-90% of themercury was retained by the gold-coated particles in the web.

Example 11

Preparation of gold-coated alumina (Al₂ O₃) particles 120 g of wide-porealumina (Scientific Adsorbents, Inc., Cat. #.0012005-99, Atlanta, Ga.)was mixed with 600 ml of 1N NaOH and heated to approximately 40° C. for20 minutes. Water was added to bring the volume of the slurry toapproximately 1500 mL and fines were removed by elutriation. Twoadditional water washes of approximately 1000 mL were used to removeadditional fines. The alumina was collected on a filter funnel andwashed with 2×600 mL water. The final wash had a pH of approximately 9to 10. Next, the alumina was washed with 2×500 mL acetone, than allowedto dry in air to a free-flowing powder.

A solution of 1..85 g AuCl₃ in a minimal amount of methyl alcohol wasadded to 600 mL of methyl t-butyl ether (MTBE), and this solution wasthen added to a slurry of the alumina in 600 mL MTBE, with constantstirring. The resulting supernatant liquid was colorless, indicatingthat all of the gold chloride was bound to the alumina.

A solution of 30 mL 37% formaldehyde, 90 mL acetone and 300 mL MTBE wasprepared and immediately added to the gold-coated alumina,, withvigorous stirring. After approximately 3 hours of stirring the aluminaparticles were a deep purple color, indicating complete reduction of thegold chloride coating to elemental gold. The supernatant solvent wasdecanted and the gold-coated alumina was collected on a filter funneland washed with 300 mL acetone followed by 300 mL methyl alcohol. Thealumina was slurried in 500 mL of 1N NaOH, collected on a funnel, washedwith 3×500 mL water, and dried in a vacuum oven at 70° C. until itbecame a free-flowing powder.

Example 12

Mercury in Air Samples: Passive Sampling; gold coated alumina particles

Gold-coated alumina particulate, prepared according to Example 11, wasincorporated into a composite membrane comprising 10% PTFE and 90%particulate, as described for sorptive particulate in U.S. Pat. No.5,071,610. Ten samples of the membrane, cut into 33 mm diameter disksand placed in 3M model 3500 Organic Vapor Monitors, were exposed tomercury vapor in air, using the mercury generation apparatus asdescribed in Example 9. Mercury and gold were eluted with HBr:HNO₃ aspreviously described, and mercury was measured by ICP spectrometry.Results are shown in Table 3, below.

                  TABLE 3                                                         ______________________________________                                               Exposure    Predicted   Measured                                       Sample time, min.  Hg/sample, μg                                                                          Hg/sample, μg                               ______________________________________                                         8     10          0.06        0.12                                            9     82          0.51        0.36                                           10     142         0.88        0.68                                           11     202         1.25        0.85                                           12     262         1.62        1.16                                           13     386         2.38        1.66                                           Blank 2                                                                              0           0           0.06                                           Blank 3                                                                              0           0           0.06                                           14     60          0.37        0.38                                           15     120         0.74        0.63                                           16     180         1.11        1.03                                           17     336         2.07        2.03                                           Blank 4                                                                              0           0           0.13                                           ______________________________________                                    

Data presented in the table shows that gold-coated alumina particles inthe microporous membrane effectively absorb mercury from air, underpassive conditions.

Example 13

Mercury in Air Samples: Dynamic Sampling; gold coated alumina particles

Single 25 mm diameter samples of gold-coated alumina-containingmembranes, as prepared in Example 12, were placed in 25 mm diameteractive sampling cassettes (Millipore Swinnex™ cassettes), and exposed tomercury vapor at a rate of approximately 1.0 liter/minute. The amount ofmercury obtained from each disk is shown in Table 4, below.

                  TABLE 4                                                         ______________________________________                                               Exposure    Predicted   Measured                                       Sample time, min.  Hg/sample, μg                                                                          Hg/sample/μg                                ______________________________________                                        18     3           0.23        0.17                                           19     10          0.81        0.73                                           20     20          1.58        1.52                                           21     45          3.69        3.86                                           Blank 5                                                                              0           0           0.00                                           Blank 6                                                                              0           0           0.03                                           ______________________________________                                    

Data in Table 4 show that gold-coated alumina particles in a microporousmembrane can effectively absorb mercury from moving air. These data alsoconfirm results presented in Table 2, above, that a second membrane isnot needed to absorb essentially all of the mercury in an activesampling device.

Example 14

One-step ionic mercury reduction and gold amalgam formation

A. Gold-coated silica particles were prepared as follows:

1. Ten grams of 35-70 micrometer diameter Davisil™ silica (300 Angstrominternal pore size), Alltech, Dearfield, Ill., were washed twice with200 mL portions of water followed by two washings with 200 mL portionsof 1M NaOH followed by three washes with 200 mL portions of water. Thewashed silica was mixed with 300 mLs of 2 percent weight/volume stannouschloride (Sigma Chemical Co., St. Louis, Mo.) and allowed to soak forabout 10 minutes. Excess stannous chloride was washed out in subsequentsteps and stannous chloride treated silica remained. The silica waswashed three times with 200 mL portions of water and then twice with 200mL portions of methanol. The methanol was removed by vacuum dryingwithout heat.

A gold solution was prepared by dissolving 150 milligrams of gold(II)chloride, Aldrich Chemical Company, Milwaukee, Wis., in 10 mLs of amixture of 9 parts (volume) isopropanol and 1 part 1M HCl (anhydrous) inethyl ether, to which was added 300 mLs of hexane.

The stannous chloride treated silica was then mixed with the goldchloride solution and allowed to stir at very slow speed until all thegold chloride was adsorbed (solution was no longer colored yellow). Thesolution was filtered and the particulate washed with two 200 mLportions of hexane followed by two 200 mL portions of isopropanol andtwo 200 mL portions of methanol. Washing was continued with three 200 mLportions of deionized water followed by two 200 mL portions withmethanol and two 200 mL portions of acetone. The purplish red goldcoated silica was vacuum dried at room temperature.

B. Stannous chloride-coated gold-coated silica particles were preparedas follows: 1.5 grams of the gold coated particles were soaked in 25 mLsof a 5 percent by weight aqueous stannous chloride solution for twohours. The coated particulate was recovered by filtering the solutionthrough Number 41 Whatman filter paper (Whatman International Ltd.,Maidstone, England). The particulate was dried at 100° C. for two hours.One cc of the dried particulate comprising elemental gold and adsorbedstannous chloride was packed into a 0.7 millimeter diameter column fortesting.

A 20 mL solution of a 1 part per million (1.00 micrograms per mL) byweight ionic mercury prepared from mercuric nitrate, SPEX Industries,Inc., Edison, N.J., and 5 percent sulfuric acid was passed through thecolumn. The resulting Effluent A was collected for subsequent analysis.The gold-mercury amalgam on the column was dissolved and eluted with 2.0mLs of a solution comprising 1.5 mLs of hydrobromic acid and 0.5 mLs ofnitric acid. This elution was followed with a deionized water wash togive a total Effluent B volume of 25 mLs for subsequent analysis.Effluents A, B, and the original sample were analyzed using ICPspectrometry as described in Example 9. No mercury was found in EffluentA indicating that the mercuric ion either was reduced by the stannouschloride to elemental mercury which was then immobilized by formation ofthe mercury-gold amalgam or mercuric ion was adsorbed by the tin salttreated particles. Effluent B contained 1.02 micrograms of mercury permL, indicating that complete recovery of ionic mercury either bysorption on the tin salt or by reduction of ionic mercury to elementalmercury and subsequent amalgamation with gold was achieved in a one-stepprocess.

Various modifications and alterations of this invention will becomeapparent to those skilled in the art without departing from the scopeand spirit of this invention, and it should be understood that thisinvention is not to be unduly limited to the illustrative embodimentsset forth herein.

We claim:
 1. A sorptive support comprising an inert, porous substrate onwhich is immobilized finely divided elemental gold-mercury amalgam, saidimmobilized gold-mercury amalgam being present in domains having amaximum dimension in the range of 1 to 100 nm, said substrate with saidimmobilized gold-mercury amalgam being enmeshed in a porous fibrous web.2. The sorptive support according to claim 1 wherein said poroussubstrate comprises silica, alumina, or zirconia.
 3. The sorptivesupport according to claim 1 wherein said porous fibrous web is selectedfrom the group consisting of polyamide, polyolefin, polyester,polyurethane, polyvinylhalide, glass, quartz, ceramic, and combinationsthereof.
 4. The sorptive support according to claim 1 wherein saidporous fibrous web comprises polytetrafluoroethylene.
 5. A sorptivesupport comprising an inert, porous substrate on which is immobilizedfinely divided high surface area elemental gold having a surface area inthe range of 5 to 100 m² /g, said immobilized gold being present indomains having a maximum dimension in the range of 1 to 100 nm, and saidsubstrate further comprising an immobilized tin salt, said substratewith said immobilized gold being enmeshed in a porous fibrous web. 6.The sorptive support according to claim 5 wherein said porous substratecomprises silica, alumina, or zirconia.
 7. The sorptive supportaccording to claim 5 wherein said porous fibrous web is selected fromthe group consisting of polyamide, polyolefin, polyester, polyurethane,polyvinylhalide, glass, quartz, ceramic, and combinations thereof. 8.The sorptive support according to claim 5 wherein said porous fibrousweb comprises polytetrafluoroethylene.
 9. A mercury monitoring devicecomprising a porous substrate on which is immobilized finely dividedhigh surface area elemental gold having a surface area in the range of 5to 100 m² /g, said immobilized gold being present in domains having amaximum dimension in the range of 1 to 100 nm, said porous substratewith said immobilized gold being enmeshed in a porous fibrous web. 10.The device according to claim 9 wherein said porous substrate comprisessilica, alumina, or zirconia.
 11. The device according to claim 9wherein said porous fibrous web is selected from the group consisting ofpolyamide, polyolefin, polyester, polyurethane, polyvinylhalide, glass,quartz, ceramic, and combinations thereof.
 12. The device according toclaim 9 wherein said porous fibrous web comprisespolytetrafluoroethylene.